Marking and Drilling

Tape the finished layout right-side up to the top (unclad) side of the board. Actually, since the layout will typically be on a sheet that is much bigger than the board itself, you will be taping the board to the back of the sheet. Alignment is much easier if you have included the board outline on the layout, since from the back you can't see the circuit portion to center it... it will be covered by the opaque board.

Place a sharp point (centerpunch, scriber, or even a sharpened nail) over each hole location and gently tap through the pattern to mark the spot. You can use a small scrap of wood for a "hammer"... a real hammer will probably be too heavy.

Next, you must drill each hole you have marked. If you are using standard fiberglass epoxy board, it will simply eat up any normal drill bit, assuming you even have one small enough. You might get away with this if you have phenolic board, or for a few (larger) mounting holes in epoxy, but in general you need CARBIDE bits for the circuit holes.

And there is a much better way than conventional drilling: Use a small high-speed hand tool like a Dremel, and use a carbide ball-tip dental burr instead of a drill bit. You can get these from dental supply houses, or maybe you can work out a deal with your dentist for the small quantity you will need.

The style of dental burr you should get has a standard 1/16 inch cylindrical shaft to fit in the corresponding collet of your Dremel tool. The shaft tapers down to where the actual ball is attached. The ball has 6 tiny curved blades or flutes.

These typically come in standard diameters that are numbered in tenths of a millimeter, so that number 010 = 1.0 mm. That is a good general-purpose size that will handle the thick leads of most large components, including TO220 power transistors and voltage regulators. But for most holes, including integrated circuit pins and standard component leads, you should use an 007 or 008 burr.

In general, you should use a hole size that just fits the leads or pins. If it's much larger the holes will take up so much space that you won't be able to fit much copper pad around them without running into adjacent traces. That would make it doubly hard to solder, since the solder would need to bridge a larger gap from the wire lead to the edge of the hole, plus there wouldn't be a good anchor for it there.

You may, however, want to get a larger burr like 012 or 013 for carving slots for larger tab-mount components like certain trimmer potentiometers.

If you have seen commercial carbide printed circuit drill bits for sale... AVOID THEM LIKE THE PLAGUE! If you try to use these in a manual setup like a Dremel tool, or even in a small Dremel drill press, they will break before you get to the third hole. The tips are too long and fragile, and any sideways force will snap them instantly... carbide is super-hard, but it's also super-brittle.

Dental burs, on the other hand, are totally forgiving: You can use them hand-held with impunity, even if you don't start the hole out straight. You can even "carve" with them to enlarge or elongate holes... no problem. (Think about what your dentist does to teeth!) They last almost forever.

Place your circuit board over a piece of scrap wood while drilling, and allow the burr to go right through into the wood a little. (Save the scrap wood for future drilling... it may end up looking like Swiss cheese, but it will still work fine for this purpose.)

Since you will be drilling tiny holes that are closely spaced, you will need good lighting. Your face will be very close to the work, so be sure to wear safey glasses or goggles. Pay attention to the air inlet and exhaust on your handpiece, to avoid blowing dust into your face. You may need to pause frequently to blow the dust from the circuit board so you can see the marks for the remaining holes.

When you are done drilling, take a moment to protect your dental burr. Although the ball tip is very hard, the shank is mild steel and will bend easily if you drop the handpiece while the burr is attached. This is about the only reason you will ever need to discard a burr... it won't "wear out".

If you can find a piece of metal tubing that is just the right diameter to fit over the end of the handpiece, you can make a slip-on cover that will shield the burr. (You can hacksaw a slit into one end of the tubing if it's too tight to fit.) Or you could make a custom wooden tool-holder for a regular workstation. But even if you have to dismount the burr from the tool, you should get in the habit of protecting it.

After all the component holes are drilled you can use a conventional low-speed drill for the few mounting holes, since they will be larger to handle standard screw sizes.

See also Printed Circuit Construction